Ion exchange resins, and particularly ion exchange resins used in water softeners, mixed bed deionizers, and the like are used to remove polyvalent ions, particularly calcium, iron, magnesium, arsenic, cadmium, lead, and other polyvalent ions that may be found in water. Thus, many homes and businesses use water softeners for the removal of such ions from the water prior to use.
When the ion exchange resin that is used in water softeners becomes exhausted, with an insufficient number of active sites available for further bonding to polyvalent ions,, the ion exchange resin is regenerated by passing brine through it. Brine is a water solution of alkali metal chloride, typically common salt. The sodium in such a brine exchanges with the polyvalent cations on the resin so that the polyvalent cations (calcium, iron, etc.) go back into solution in the brine, being replaced on the resin by sodium ions from the brine. In the past, the spent brine containing such polyvalent cations was then simply discarded down the drain.
However, in many areas, environmental regulations are placing strong constraints upon the nature of the solutions which may be disposed into drains or the ground waters. In arid areas such as California, rigid restrictions are coming into place, providing strong constraints on particularly commercial users of water softners, creating a need to find new ways to dispose of the spent brine. Hence, there is a need for particularly large scale users of brine to reduce the amounts of waste solutions for disposal. Currently, in some areas, such waste brine has to be trucked to a permitted disposal site, or it has to be diluted with tap water before it can be disposed down the drain, both of which techniques are expensive and wasteful of resources.
Recently, as described in the article by Howard Conner entitled Brine Recovery, (Water Technology, Jul., 1992 pages 34 and 35) techniques for recovery of usable brine from spent brine are described. As stated, . in traditional brine reclaiming, the first third of the brine recovered in an ion exchange regeneration process is sent down the drain. The middle third is stored for later use. The final third, containing rinse water, has the lowest ion concentration, and may be also sent down the drain, being typically of acceptable quality for disposal. However, such a technique is not helpful in areas where regulations strongly constrain the disposal of water solutions having higher ionic concentrations such as the first brine third mentioned above.
The above cited article also describes the brine recovery process in which the hardness of the brine (i.e., polyvalent ions) is removed by "nanofiltration", in which membranes are used under conditions in which the sodium chloride of the brine tends to pass through the membrane, while the polyvalent ions are rejected. Sulfuric acid is added to the spent brine for the nanofiltration process, which results in an improvement in the rejection of polyvalent ions. Thus, the rejected component of the solution, which is only a fraction of the volume of the original spent brine, presents a reduced disposal problem while the purified brine, having greatly reduced numbers of polyvalent ions, can be reused, for example in the regeneration of an ion exchange resin.
Thus the quantity of waste product, namely the rejected component of the spent brine, can be disposed of with greater ease and less cost, since its volume is only a fraction of that of the original spent brine. Furthermore, significant savings on both salt and water consumption are provided.
In accordance with this invention, the above process is improved to preferably reduce the cost thereof, and also to reduce the risk of precipitation of solutes in the spent brine prior to nanofiltration. Such precipitated materials can cause clogging and scaling on the surface of the nanofiltration membrane, and thus should be avoided. Particularly, in spent brines where the calcium ion content is high, the sulfuric acid present can result in the precipitation of calcium sulfate, with resulting undesirable results of clogging or scaling, often, an antiscaling agent must be added to the spent brine before nanofiltration in the prior art manner.